Fix stringish::simplified() for strings > 2Gi characters

If, after simplification, the result string would have been >2Gi
characters in length, the code returned a shorter string, truncated
mod INT_MAX, because the algorithm had not been ported to qsizetype.

Task-number: QTBUG-103531
Change-Id: I57cfed34e5c69406f8d3f2913962d006b4bc6bd0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit ec35efd7f2cea4dcbe0188f0d44cde541803f53e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qstringalgorithms_p.h

@@ -101,7 +101,7 @@ template <typename StringType> struct QStringAlgorithms
         if (ptr != dst && ptr[-1] == QChar::Space)
             --ptr;
 
-        int newlen = ptr - dst;
+        qsizetype newlen = ptr - dst;
         if (isConst && newlen == str.size() && unmodified) {
             // nothing happened, return the original
             return str;